Leflunomide is a synthetic isoxazol or, more specific: 5-methyl-N-[4- (trifluoromethyl)phenyl]-4-isoxazolecarboxamide (figure 1) and it is a crystalline solid. [1] Leflunomide is a drug used for the treatment of rheumatoid arthritis, an illness that affects soft tissues and bones and can cause irreversible joint deformities and functional impairment. [2] Leflunomide has an antiproliferative action. It acts as a dihydroorotate dehydrogenase inhibitor as is has been shown to be selectively inhibited by the active metabolite of leflunomide. [2] This drug is administered orally, and its bioavailability is aprox. 80%. In spite of its rather high bioavailability, leflunomide is practically insoluble in water (less than 40 mg/L), so it belongs to class II of the biopharmaceutics classification systems. [2] TG, DSC and FT-IR studies have been used to highlight the degree of incorporation of the active substance in the membrane and the possible interactions between the drug and the polymer.figureFigure.1. Chemical structure of leflunomide Figure 2. Normalized Heat Flow for membrane and membrane impregnated with leflunomide 10 respectivelly 30 mins